1. Field of the Invention
The present invention relates generally to optical filtering systems and more particularly to an improved filtering system for controlling the color of light projected by a spot light or the like.
2. Discussion of the Prior Art
Conventional lighting systems have been provided with replaceable filters for modulating the color and/or intensity of light projected through the filters. U.S. Pat. No. 3,411,847 describes a photographic film developing system which passes a beam of white light through filters which each uniformly attenuates a respective primary color. The filters are individually positioned to be intersected by the beam during fractions of a film developing interval which are proportional to the fraction of that color in the average desired color of the light over the entire interval. The filtered light is differently colored during different fractions of the interval and typically at no one instant has the desired average color.
U.S. Pat. No. 4,459,014 describes a system of three unspecified or uniform density primary color filters which are individually and constantly positioned in parallel so that they are intersected by light in a proportional cross-sectional area of a beam while white light passes in the unfiltered remaining area of the beam. The downstream beam has the desired average color but its hue and intensity is non-uniformly distributed and inefficient to diffuse.
U.S. Pat. No. 4,600,976 describes a comparable mechanism in which multiple unspecified (uniform) density primary color filters are individually rotatable in parallel planes to be intersected by proportional areas of a light beam, and thereby yield a downstream beam that also has the desired overall average color but with a non-uniform intensity distribution, which again requires substantial diffusion.
In U.S. Pat. No. 4,602,321 each of three sets of filters include filter elements with respective planar surfaces which are uniformly covered with dichroic filtering material. The filter elements are rotatable around axes in their respective surface planes to positions oblique to the optical axis of the beam. This produces a downstream beam with a selected one of various colors distributed generally uniformly. However, dichroic filters being rotated about axes non-normal to their surfaces differently affect different spectra of light along the beam axis. This limits the possible colors and/or intensities producible by this technique with given sets of color filter elements.
There remains therefore a need for a spectral filter for selectively, continuously, uniformly, and conveniently varying the spectral composition of a beam of light projected through the filter.